The present invention broadly relates to a new and improved construction of a measuring probe for determining at least one volatile constituent which is contained in a liquid medium and which can be chemically converted with an accompanying heat effect.
In its more particular aspects, the present invention specifically relates to a new and improved construction of a measuring probe for determining at least one volatile constituent which is contained in a liquid medium and which can be chemically converted with an accompanying heat effect, and which measuring probe contains a tubular casing or housing closed on one side by a membrane which is permeable for the at least one volatile constituent. In its interior, the tubular casing or housing contains a concentrically arranged internal body or member and a detector which responds to temperature changes and which constitutes a pellistor. The pellistor is constructed from an electric heating element, specifically a platinum wire which can be connected to a power source, and a matrix into which the electrical heating element is embedded. The pellistor further contains a surface coating containing an oxidation catalyst. The nature of the pellistor is such that the pellistor emits or generates electrical measuring signals as a function of the aforementioned temperature changes.
When carrying out microbiological or food technology processes, it is frequently necessary for monitoring and controlling such processes, to determine the content of volatile constituents, particularly lower alcohols such as methanol and ethanol in the medium under investigation. The content of these constituents provides information, for example, with respect to the progress of a fermentation process or the alcohol content of a liquid, for example, a beverage.
The determination of alcohols, particularly ethanol, as volatile constituents of culture media in fermenters hitherto generally was effected using gas chromatography, mass spectrometry or by means of flame ionization detectors. The amount of time and work necessary for carrying out these determinations is relatively great so that there exists a considerable demand for simpler and less costly methods.
It is further known to flush the volatile constituents out of the fermenter by means of a carrier gas, for example, air or nitrogen and pass the thus obtained gas mixture along a heated platinum wire coil. During such passage, the volatile constituents, for example, alcohol are subjected to oxidation.
It is also known to employ semiconductor elements for determining oxidizable gases in containers for combustible liquids, for example, in fuel tanks. Such semiconductor elements experience a resistance change due to the temperature change caused by the absorption of the volatile constituents at the semiconductor surface. However, due to the insufficient linearity and the complicated pre-treatment or preconditioning, it is impractical to use such semiconductor elements as detectors for the quantitative determination of alcohols present in the liquid phase.
In addition, there are known gas sensors which contain as the essential component a pellistor in which a metallic filament is surrounded by a metal oxide coated and/or impregnated with a catalyst material. These gas sensors are mainly used for monitoring the air and are sensitive to the action of thermal shocks and condensate formation so that their use is limited to a relatively narrow field.